1. Field of the Invention
This invention broadly relates to a method for recovering for subsequent use Group VIII transition metals from a non-polar organic solution containing a coordination complex a Group VIII transition metal and an organo-substituted ligand of a trivalent atom of a Group VA element including phosphorus, arsenic and antimony, e.g., an organophosphorus ligand. This invention particularly relates to a method for recovering rhodium from a solution of a rhodium-organophosphorus ligand coordination complex in a non-polar solvent used in the hydroformylation of olefinic compounds.
2. Description of Related Art
Processes using transition metal-ligand complexes as homogeneous catalysts are well-known. Included in such processes are the hydrogenation of unsaturated compounds, the carbonylation of methanol to acetic acid, olefin dimerization and oligomerization processes, the hydrocyanation of butadiene to adiponitrile and olefin hydrosilylation reactions. Still other processes are known to those skilled in the art. Oftentimes, recovery of the transition metal from the catalyst solutions used in these process presents a particularly troublesome problem.
Particularly illustrative of these homogeneous catalysis systems is the hydroformylation of olefinic compounds with carbon monoxide and hydrogen to produce aldehydes in the presence of a coordination complex of a Group VIII transition metal and an organophosphorus ligand dissolved in an organic (non-polar) solvent. U.S. Pat. No. 3,527,809, for example, teaches selectively hydroformylating alphaolefins with certain rhodium-triorganophosphine and triorganophosphite ligand catalyst complexes to produce oxygenated products rich in normal aldehydes. U.S. Pat. Nos. 4,148,830 and 4,247,486 disclose hydroformylation processes which use rhodium triarylphosphine ligand catalyst complexes. U.S. Pat. No. 4,283,562 discloses that branched-alkyl-phenylphosphine or cycloalkylphenylphosphine ligands can be employed in a rhodium catalyzed hydroformylation process in order to provide a more stable catalyst. U.S. Pat. No. 4,400,548 discloses that bisphosphine monoxide ligands can be employed for hydroformylation and such ligands provide rhodium catalyst complexes of improved thermal stability. Other patents describing hydroformylation processes and catalysts include U.S. Pat. Nos. 4,599,206; 4,668,651; 4,717,775; 4,737,588; and 4,748,261.
Rhodium complex catalyzed hydroformylation processes preferably are carried out in a non-aqueous hydroformylation reaction medium containing an organic (non-polar) solvent and both organic solvent-soluble catalyst complex and soluble free ligand, i.e., ligand not tied to or bound to the rhodium catalyst complex. Organic (non-polar) solvents which do not interfere with the hydroformylation process can be employed. Included in the group of suitable non-polar organic solvents are compounds belonging to the general classes of alkanes, ethers, aldehydes, ketones, esters, amides, and aromatics.
Aldehyde compounds corresponding to the desired aldehyde products and especially higher boiling liquid aldehyde condensation by-products (oligomers) produced in situ during the hydroformylation process are particularly useful as non-polar organic solvents in rhodium-catalyzed hydroformylation. In this regard, the aldehyde product and the corresponding aldehyde trimers are preferred for start-up of a continuous hydroformylation process. However, as hydroformylation proceeds, the solvent typically will comprise both aldehyde products and higher boiling liquid aldehyde condensation by-products due to the nature of such continuous processes. Methods for the preparation of such aldehyde condensation by-products are more fully described in U.S. Pat. Nos. 4,148,830 and 4,247,486.
It may be preferred to separate and recover the desired aldehyde product from the non-polar hydroformylation reaction medium containing the catalyst complex by vaporization or distillation. Hydroformylation systems using both continuous gas recycle and liquid recycle are known. In a continuous liquid catalyst recycle operation such as described in U.S. Pat. No. 4,148,830, liquid aldehyde condensation by-products including aldehyde trimers and higher oligomers, produced under hydroformylation conditions from the desired aldehyde product, are employed as the reaction solvent for the catalyst. This process has been used widely to hydroformylate lower olefinic compounds containing from two to five carbon atoms, particularly lower alpha-olefins, to produce aldehydes containing from three to six carbon atoms. In general, it is preferred to separate the desired aldehyde product from the rhodium containing product solution by selective vaporization of the aldehyde under reduced pressure and at temperatures below about 150.degree. C., preferably below about 130.degree. C.
Commercial experience has shown that the rhodium catalyst complexes used in hydroformylating olefinic compounds are deactivated by the presence in the feedstock of extrinsic catalyst poisons, such as sulphurous compounds (e.g. H.sub.2 S, COS and CH.sub.3 SH) or halogen compounds (e.g. HCl), which can react with the rhodium of the catalyst to form inactive species which are not destroyed under the mild hydroformylation conditions employed. Hence great care is taken to purify the various feedstocks. Deactivation of the rhodium hydroformylation catalyst also occurs, however, even in the substantial absence of extrinsic poisons. This deactivation is referred to as intrinsic deactivation and is believed to be due, inter alia, to the effects of temperature, reactant partial pressures, specific organophosphorus ligands employed, and the rhodium concentration. The extent of catalyst deactivation (or the catalyst activity) is determined at any particular time by comparing the conversion rate of reactants to aldehyde product at that particular time, to the conversion rate obtained using fresh catalyst.
Another potential problem for continuous hydroformylation processes is the accumulation of aldehyde condensation by-products of low volatility relative to the organophosphorus ligand. Since commercial processes rely on vaporization or distillation to separate the reaction product medium containing the rhodium-ligand complex from the desireed aldehyde hydroformylation products, the accumulation of high boiling aldehyde condensation by-products having a low volatility must be accounted for when designing the hydroformylation process.
When continiuously hydroformylating lower olefinic compounds, the accumulation rate of high boiling aldehyde condensation by-products normally is sufficiently low that it can be easily controlled. Thus, in the case of hydroformylating lower olefins, catalyst life mainly is limited by the rate of catalyst deactivation. In present commercial plants, it is not unusual to operate the process for upwards of one year or more during which time any decline in catalyst activity or catalyst loss may be easily offset by addition of fresh catalyst or catalyst precursor, together with fresh organophosphorus ligand if desired. However, in such systems when the level of deactivated rhodium species rises to undesirable values and it is no longer considered economical to continue the hydroformylation process, it may become expedient simply to replace the catalyst charge completely, even though it may still contain a significant proportion of active rhodium catalyst complex. Because rhodium is and expensive metal, however, it is uneconomical to discard the spent catalyst and a usual practice is for the catalyst to be reactivated. U.S. Pat. Nos. 4,297,239; 4,374,278 and 4,446,074, for example, describe procedures for reprocessing and reactivating spent hydroformylation catalyst.
Continuous hydroformylation of higher olefinic compounds, such as higher alpha-olefins, containing six or more carbon atoms, e.g., from six to thirty carbon atoms, using conventional organic (non-polar) solvent-solubilized organophosphorus ligands, involves a further problem since aldehyde condensation by-products, having a low volatility relative to the organophosphorus ligands, accumulate at a much higher rate than encountered during the hydroformylation of lower olefins. Unfortunately the high rate of accumulation of such high boiling aldehyde condensation by-products cannot be readily controlled by removing same by distillation from the catalyst solution during the hydroformylaton without incurring significant energy costs and exposing the catalyst to severe temperature conditions. Thus, it may be more practical to let such hydroformylations of higher olefins to merely run their course until the accumulation of such by-products becomes so great as to overwhelm the catalyst solution and economically prevent its further usefulness, at which time the catalyst solution needs to be replaced with a fresh catalyst solution.
There remains, however, the additional problem of recovering the rhodium values, much of which may still be highly active, from said overwhelmed catalyst solution that is replaced. In fact, the hydroformylation catalyst still may exhibit over 75% and possibly even over 90% of its initial catalytic activity at this time. Again, unfortunately, due to their low volatility, the removal of such by-products from the replaced hydroformylation catalyst solution using conventional distillation techniques is plagued by the same energy-related and thermal exposure problems referenced above. Thus, there is a need in the art for a less energy intensive method for recovering rhodium from such catalyst solutions under more mild temperature conditions than possible using distillation procedures.
Thus, it is an object of the present invention to provide a method for recovering Group VIII transition metals and particularly rhodium from substantially non-polar organic solutions containing coordination complexes of the transition metals and organo-substituted ligands of a trivalent atom of a Group VA element such as organophosphorus ligands. Such a method potentially has application in the wide variety of homogeneous catalysis applications noted earlier, and particularly for removing rhodium from substantially non-polar organic solutions in which large concentrations of hard-to-remove components have accumulated, such as the high boiling aldehyde condensation by-products that accumulate during the hydroformylation of higher olefinic compounds.